Irrigation drippers having screw threaded shanks with central axial bores are known in U.S. Pat. Nos. 1,520,048 (Baird); 3,335,964 (Singleton); 4,199,105 (Olson et al) and 3,815,831 (Jooste). Some of the drippers described in these patents have arrangements for adjustment of the rate of delivery of water while in others there is no provision for adjustments to the delivery volume. A disadvantage of the known drippers is that the volume of water delivered cannot be varied or in instances where provision is made to vary the delivery volume, the arrangements are elaborate. Further, it is difficult in use to clean the blocked internal passage in the shanks.